Air infiltration. The amount of air
leaking in and out of a building through cracks in walls, windows and doors.

Air-leakage (air
infiltration).The amount of air leaking in and out of a building through cracks in
walls, windows, and doors.

Air-leakage rating. A measure of the rate of
air-leakage around a window, door, or skylight in the presence
of a specific pressure difference. It is expressed in
units of cubic feet per minute per square foot of frame
area (cfm/sq ft). Formerly expressed as cubic feet
per minute per foot of window perimeter length
(cfm/ft) but not now in use. The lower a window's
air-leakage rating, the better its airtightness.

Annealed glass. Standard sheet of float glass which
has not been heat-treated.

Annealing. Heating above the critical or
recrystallization temperature, then controlled cooling of metal,
glass, or other materials to eliminate the effects of
cold-working, relieve internal stresses, or improve
strength, ductility, or other properties.

ANSI. American National Standards Institute.
Clearing house for all types of standards and specifications.

B

Balance. A mechanical device (normally
spring-loaded) used in single- and double-hung windows as a
means of counterbalancing the weight of the sash
during opening and closing.

Bay window. An arrangement of three or more
individual window units, attached so as to project
from the building at various angles. In a three-unit bay,
the center section is normally fixed, with the end
panels operable as single-hung or casement windows.

Bead. A wood strip against which a swinging sash
closes, as in a casement window. Also, a finishing trim at
the sides and top of the frame to hold the sash, as in a
fixed sash or a double-hung window. Also referred to
as bead stop.

Blackbody. The ideal, perfect emitter and absorber
of thermal radiation. It emits radiant energy at
each wavelength at the maximum rate possible as a
consequence of its temperature, and absorbs all
incident radiance.

BOCA. Building Officials and Code Administrators.

Bottom rail. The bottom horizontal member of a
window sash.

Bow window. A rounded bay window that projects
from the wall in an arc shape, commonly consisting of
five sashes.

Brick molding. A standard milled wood trim piece
that covers the gap between the window frame and
masonry.

Btu (B.T.U.). An abbreviation for British Thermal
Unit, the heat required to increase the temperature of
one pound of water one degree Fahrenheit.

C

Casement. A window sash that swings open on
side hinges: in-swinging are French in origin;
out-swinging are from England.

Casing. Exposed molding or framing around a window
or door, on either the inside or outside, to cover the
space between the window frame or jamb and the wall.

Caulking. A mastic compound for filling joints and
sealing cracks to prevent leakage of water and
air, commonly made of silicone, bituminous, acrylic,
or rubber-based material.

CFM. Cubic Feet per Minute.

Check rail. The bottom horizontal member of the
upper sash and the top horizontal member of the lower
sash which meet at the middle of a double-hung window.

Clerestory. A window in the upper part of a lofty
room that admits light to the center of the room.

Composite frame. A frame consisting of two or
more materials for example, an interior wood
element with an exterior fiberglass element.

Condensation. The deposit of water vapor from the air
on any cold surface whose temperature is below the
dew point, such as a cold window glass or frame that
is exposed to humid indoor air.

Conduction. Heat transfer through a solid material
by contact of one molecule to the next. Heat flows from
a higher-temperature area to a lower-temperature one.

Convection. A heat transfer process involving motion
in a fluid (such as air) caused by the difference in
density of the fluid and the action of gravity.
Convection affects heat transfer from the glass surface to room
air, and between two panes of glass.

CRF. Condensation Resistance Factor. An indication of
a window's ability to resist condensation. The
higher the CRF, the less likely condensation is to
occur. Based on AAMA standard.

D

Degree day. A unit that represents a one-degree
Fahrenheit deviation from some fixed reference point
(usually 65° F) in the mean, daily outdoor temperature. See also heating degree day.

Desiccant. An extremely porous crystalline
substance used to absorb moisture from within the sealed
air space of an insulating glass unit.

Dewpoint. The temperature at which water vapor in
air will condense at a given state of humidity and
pressure.

Divided light. A window with a number of smaller
panes of glass separated and held in place by muntins.

DOE-2.1E. A building-simulation computer program
used to calculate total annual energy use.

Double glazing. In general, two thicknesses of
glass separated by an air space within an opening to
improve insulation against heat transfer and/or
sound transmission. In factory-made double glazing
units, the air between the glass sheets is thoroughly
dried and the space is sealed airtight, eliminating
possible condensation and providing superior insulating
properties.

Double-hung window. A window consisting of two
sashes operating in a rectangular frame, in which both
the upper and lower halves can be slid up and down.
A counterbalance mechanism usually holds the sash
in place.

E

Edge effects. Two-dimensional heat transfer at the
edge of a glazing unit due to the thermal properties
of spacers and sealants.

Electrochromics. Glazing with optical properties
that can be varied continuously from clear to dark with
a low-voltage signal. Ions are reversibly injected
or removed from an electrochromic material,
causing the optical density to change.

Electromagnetic spectrum. Radiant energy over a broad range of wavelengths.

Emergency exit window. Fire escape window
(egress window) large enough for a person to climb out.
In U.S. building codes, each bedroom must be
provided with an exit window. The exact width, area,
and height from the floor are specified in the
building codes.

Emittance. The ratio of the radiant flux emitted by
a specimen to that emitted by a blackbody at the
same temperature and under the same conditions.

Evacuated glazing. Insulating glazing composed of
two glass layers, hermetically sealed at the edges, with
a vacuum between to eliminate convection and conduction. A spacer system is needed to keep the
panes from touching.

Exterior stop. The removable glazing bead that holds
the glass or panel in place when it is on the exterior
side of the light or panel, in contrast to an interior
stop located on the interior side of the glass.

Extrusion. The process of producing vinyl or
aluminum shapes by forcing heated material through an
orifice in a die. Also, any item made by this process.

Eyebrow windows. Low, inward-opening windows
with a bottom-hinged sash. These attic windows built
into the top molding of the house are sometimes
called "lie-on-your-stomach" or "slave" windows.
Often found on Greek Revival and Italianate houses.

F

Fanlight. A half-circle window over a door or
window, with radiating bars. Also called circle top
transom.

Fenestration. The placement of window openings in
a building wall, one of the important elements in
controlling the exterior appearance of a building. Also,
a window, door, or skylight and its associated
interior or exterior elements, such as shades or
blinds.

Fiberglass. A composite material made by
embedding glass fibers in a polymer matrix. May be used as
a diffusing material in sheet form, or as a standard
sash and frame element.

Fixed light. A pane of glass installed directly into
non-operating framing members; also, the opening
or space for a pane of glass in a non-operating frame.

Fixed panel. An inoperable panel of a sliding glass
door or slider window.

Fixed window. A window with no operating sashes.

Flashing. Sheet metal or other material applied to
seal and protect the joints formed by different materials
or surfaces.

Float glass. Glass formed by a process of floating
the material on a bed of molten metal. It produces a
high-optical-quality glass with parallel surfaces,
without polishing and grinding.

Fogging. A deposit of contamination left on the
inside surface of a sealed insulating glass unit due to
extremes of temperatures or failed seals.

Frame. The fixed frame of a window which holds the
sash or casement as well as hardware.

Heat gain. The transfer of heat from outside to inside
by means of conduction, convection, and radiation through all surfaces of a house.

Heating degree day. Term used by heating and
cooling engineers to relate the typical climate conditions
of different areas to the amount of energy needed to
heat and cool a building. The base temperature is 65
degrees Fahrenheit. A heating degree day is counted
for each degree below 65 degrees reached by the
average daily outside temperatures in the winter. For
example, if on a given winter day, the daily
average temperature outdoors is 30 degrees, then there are
35 degrees below the base temperature of 65
degrees. Thus, there are 35 heating degree days for that day.

Heat loss. The transfer of heat from inside to outside
by means of conduction, convection, and radiation through all surfaces of a house.

Heat-strengthened glass. Glass that is reheated, after forming, to just below melting point, and then
cooled, forming a compressed surface that increases
its strength beyond that of typical annealed glass.

Hinged windows. Windows (casement, awning,
and hopper) with an operating sash that has hinges on
one side. Seealso Projected window.

Hopper. Window with sash hinged at the bottom.

Horizontal slider. A window with a movable panel
that slides horizontally.

Hybrid Tubular Daylighting Device (HTDD). A tubular daylighting device (TDD) whose
light transmitting tube consists of more than one material and/or has more than one
geometry throughout its length. Typically used with suspended ceilings or to illuminate
spaces without ceilings.

I

ICC. International Code Council. A national
organization that publishes model codes for adoption by states
and other agencies. Codes include the International
Building Code (IBC) and the International Energy Conservation Code (IECC).

IECC. International Energy Conservation Code
published by the ICC. The successor to the Model Energy
Code, which is cited in the 1992 U.S. Energy Policy
Act (EPAct) as the baseline for residential Energy
Codes in the United States.

K

L

Laminated glass. Two or more sheets of glass with
an inner layer of transparent plastic to which the
glass adheres if broken. Used for safety glazing and
sound reduction.

Lift. Handle for raising the lower sash in a
double-hung window. Also called sash lift.

Light. A window; a pane of glass within a
window. Double-hung windows are designated by the
number of lights in upper and lower sash, as in
six-over-six. Also spelled informally lite.

Light-to-solar-gain ratio. A measure of the ability of a glazing to provide light without excessive solar
heat gain. It is the ratio between the visible
transmittance of a glazing and its solar heat gain coefficient.
Abbreviated LSG.

Lintel. A horizontal member above a window or
door opening that supports the structure above.

Liquid crystal glazing. Glass in which the optical
properties of a thin layer of liquid crystals are controlled
by an electrical current, changing from a clear to
a diffusing state.

Low-conductance spacers. An assembly of
materials designed to reduce heat transfer at the edge of
an insulating window. Spacers are placed between
the panes of glass in a double- or triple-glazed window.

M

Meeting rail. The part of a sliding glass door, a
sliding window, or a hung window where two panels
meet and create a weather barrier.

Metal-clad windows. Exterior wood parts covered
with extruded aluminum or other metal, with a
factory-applied finish to deter the elements.

Micron. One millionth
(10-6) of a metric meter.

Mil. One thousandth of an inch, or 0.0254 millimeter.

Model Energy Code (MEC). The Model Energy Code is cited in the 1992 U.S. Energy Policy Act (EPAct) as
the baseline for residential Energy Codes in the
United States. It has been succeeded by the
International Energy Conservation Code (IECC) published by
the International Code Council (ICC).

Mullion. A major structural vertical or horizontal
member between window units or sliding glass doors.

Muntin. A secondary framing member (horizontal,
vertical, or diagonal) to hold the window panes in
the sash. This term is often confused with mullion.

Muntin grilles. Wood, plastic, or metal grids designed
for a single-light sash to give the appearance of
muntins in a multilight sash, but removable for ease in
cleaning the window.

P

Pane. One of the compartments of a door or
window consisting of a single sheet of glass in a frame; also,
a sheet of glass.

Panel. A major component of a sliding glass door,
consisting of a light of glass in a frame installed within
the main (or outer) frame of the door. A panel may
be sliding or fixed.

Panning. In replacement window work, the outside
aluminum trim that can extend around the perimeter
of the window opening; used to cover up the old
window material. Panning can be installed in the
opening before the window, or can be attached directly to
the window before installation.

Particle dispersed glazing. Glazing in which the orientation of small particles between two sheets of glass
is controlled electrically, thus changing its optical
properties.

Parting stop. A narrow strip, either integral or
applied, that holds a sash or panel in position in a frame.

Peak load. The maximum thermal load to be provided
by a heating or cooling system in a house.

Photochromics. Glazing with the optical properties
that change in response to the amount of incident light.

Picture window. A large, fixed window framed so that
it is usually, but not always, longer horizontally
than vertically to provide a panoramic view.

Pivot window. A window with a sash that swings open
or shut by revolving on pivots at either side of the sash
or at top and bottom.

Plastic film. A thin plastic substrate, sometimes used
as the inner layers in a triple- or quadruple-glazed
window.

Plastics. Artificial substances made of organic
polymers that can be extruded or molded into various
shapes including window frames and sashes.

Polyvinylchloride (PVC). An extruded or molded
plastic material used for window framing and as a
thermal barrier for aluminum windows.

Projected window. A window fitted with one or
more sashes opening on pivoted arms or hinges. Refers
to casements, awnings, and hoppers.

Pyrolytic low-E. A low- E coating which typically
uses tin oxide with some additives deposited directly onto a glass
surface while it is still hot. The result is a baked-on surface layer
that is hard and durable and thus sometimes referred to as a "hard
coat." Pyrolytic coatings are typically used in insulated glass units
with the low-E surface inside the sealed air space, but can also be
applied to single-pane glass and separate storm windows.

R

R-value. A measure of the resistance of a glazing
material or fenestration assembly to heat flow. It is the
inverse of the U-factor (R = 1/U) and is expressed in units
of hr-sq ft-°F/Btu. A high-R-value window has a
greater resistance to heat flow and a higher insulating
value than one with a low R-value.

Radiation. The transfer of heat in the form of
electromagnetic waves from one separate surface to
another. Energy from the sun reaches the earth by
radiation, and a person's body can lose heat to a cold window
or skylight surface in a similar way.

Refraction. The deflection of a light ray from a
straight path when it passes at an oblique angle from
one medium (such as air) to another (such as glass).

Relative humidity. The percentage of moisture in the
air in relationship to the amount of moisture the air
could hold at that given temperature. At 100 percent
relative humidity, moisture condenses and falls as rain.

S

Safety glass. A strengthened or reinforced glass that
is less subject to breakage or splintering.

Sash. The portion of a window that includes the glass
and the framing sections directly attached to the glass,
not to be confused with the complete frame into which
the sash sections are fitted.

Screen. Woven mesh of metal, plastic, or
fiberglass stretched over a window opening to permit air to
pass through, but not insects.

Sealant. A compressible plastic material used to seal
any opening or junction of two parts, such as between
the glass and a metal sash, commonly made of
silicone, butyl tape, or polysulfide.

Shade screen. A specially fabricated screen of
sheet material with small narrow louvers formed in
place to intercept solar radiation striking a window;
the louvers are so small that only extremely small
insects can pass through. Also called sun
screen. Also, an awning with fixed louvers of metal or wood.

Shading coefficient
(SC). A measure of the ability of a window or skylight to transmit solar heat, relative
to that ability for 1/8-inch clear, double- strength,
single glass. It is being phased out in favor of the solar
heat gain coefficient, and is approximately equal to
the SHGC multiplied by 1.15. It is expressed as a
number without units between 0 and 1. The lower a
window's solar heat gain coefficient or shading coefficient,
the less solar heat it transmits, and the greater is
its shading ability.

Sheet glass. A transparent, flat glass found in older
windows, now largely replaced by float glass.

Sill track. The track provided at the sill of a sliding
glass door. Also, the sill member incorporating such
a track.

Simulated divided lights. A window that has the
appearance of a number of smaller panes of glass
separated by muntins, but actually is a larger glazing unit
with the muntins placed between or on the surfaces of
the glass layers.

Single glazing. Single thickness of glass in a window
or door.

Single-hung window. A window consisting of two
sashes of glass, the top one stationary and the bottom
movable.

Skylight (operable or pivot). A roof window that gives light and ventilation.

Sliding glass door. A door fitted with one or more
panels that move horizontally on a track and/or in
grooves. Moving action is usually of rolling type (rather
than sliding type). Also called gliding door, rolling
glass door, and patio sliding door.

Sliding window. A window fitted with one or
more sashes opening by sliding horizontally or vertically
in grooves provided by frame members. Vertical
sliders may be single- or double-hung.

Smart window. Generic term for windows with
switchable coatings to control solar gain.

Solar control coatings. Thin film coatings on glass
or plastic that absorb or reflect solar energy,
thereby reducing solar gain.

Solar heat gain coefficient (SHGC). The fraction of solar radiation admitted through a window or
skylight, both directly transmitted, and absorbed and
subsequently released inward. The solar heat gain
coefficient has replaced the shading coefficient as the
standard indicator of a window's shading ability. It is
expressed as a number between 0 and 1. The lower a
window's solar heat gain coefficient, the less solar heat it
transmits, and the greater its shading ability. SHGC can
be expressed in terms of the glass alone or can refer to
the entire window assembly.

Solar radiation. The total radiant energy from the
sun, including ultraviolet and infrared wave lengths
as well as visible light.

Solar spectrum. The intensity variation of sunlight
across its spectral range.

Sound Transmission Class (STC). The sound transmission loss rating of a material over a selected range
of sound frequencies. The higher the number, the
less sound transmitted.

Spectrally selective glazing. Glazing that is
transparent to some wavelengths of the solar spectrum and reflective to
others. Typical spectrally selective coatings are transparent to visible
light and reflect short-wave and long-wave infrared as well as UV
radiation. Spectrally selectivity can be achieved with low-E coatings
and/or high-performance tints.

Stile. The upright or vertical edges of a door, window,
or screen.

Sputtered low-E. A multi-layered low-E coating deposited on glass or
plastic film. Typically consists of three primary layers with at least
one layer of metal. Sputtered coatings often use one, two or three
silver layers and must be protected from humidity and contact. For this
reason, they are often referred to as "soft coat." Sputter coatings are
typically used in insulated glass units with the low-E surface inside
the sealed air space, but can also be applied to plastic and used in
suspended films or retrofit window films.

Stool. The shelf-like board of the interior part of
the window sill, against which the bottom rail of the
sash closes.

Stop. The molding on the inside of a window
frame against which the window sash closes; in the case
of a double-hung window, the sash slides against
the stop. Also called bead, side stop, window stop, and parting stop.

Storm windows. A second set of windows installed on
the outside or inside of the primary windows to
provide additional insulation and wind protection.

Sun control film. A tinted or reflective film applied to
the glazing surface to reduce visible, ultra-violet, or
total transmission of solar radiation. Reduces solar
heat gain in summer and glare. Some can be removed
and reapplied with changing seasons.

Superwindow. A window with a very low U-factor,
typically less than 0.15, achieved through the use
of multiple glazings, low-E coatings, and gas fills.

Switchable glazings. Glazings with optical
properties that can be reversibly switched from clear to dark
or reflective.

T

Tempered glass. Treated glass that is strengthened
by reheating it to just below the melting point and
then suddenly cooling it. When shattered, it breaks
into small pieces. Approximately five times stronger
than standard annealed glass; is required as safety
glazing in patio doors, entrance doors, side lights, and
other hazardous locations. It cannot be recut after
tempering.

Thermal break. An element of low conductance
placed between elements of higher conductance to
reduce the flow of heat. Often used in aluminum windows.

Thermal expansion. Change in dimension of a
material as a result of temperature change.

Thermal mass. Mass in a building (furnishings or
structure) that is used to absorb solar gain during the
day and release the heat as the space cools in the evening.

Thermochromics. Glazing with optical properties
that can change in response to temperature changes.

Thermogram. An image of an object taken with an
infrared camera that shows surface temperature variations.

Threshold. The member that lies at the bottom of
a sliding glass door or swinging door; the sill of
a doorway.

Tilt window. A single- or double-hung window
whose operable sash can be tilted into the room for
interior washability.

Tinted glass. Glass colored by incorporation of a
mineral admixture. Any tinting reduces both visual and
radiant transmittance.

Transmittance. The percentage of radiation that
can pass through glazing. Transmittance can be
defined for different types of light or energy, e.g., visible
light transmittance, UV transmittance, or total solar
energy transmittance.

Transom. A horizontal transverse beam or bar in
a frame; a crosspiece separating a door or the like
from a window or fanlight above it. Also, a window
above a door or other window, built on and
commonly hinged to a transom.

Transom window. The window sash located above
a door. Also called transom light.

Triple glazing. Three panes of glass or plastic with
two air spaces between.

Tubular Daylighting Device (TDD). A non-operable device primarily designed to transmit
daylight from a roof surface to an interior ceiling surface via a tubular conduit. The device
consists of an exterior glazed weathering surface, a light transmitting tube with a reflective
inside surface and an interior sealing device, such as a translucent ceiling panel. See also
Hybrid Tubular Daylighting Device.

U

U-factor (U-value). A measure of the
rate of non-solar heat loss or gain through a material or assembly. It
is expressed in units of Btu/hr-sq ft-°F (US) or W/sq m-°K
(European metric). Values are normally given for NFRC/ASHRAE winter conditions
of 0° F (18° C) outdoor temperature, 70° F (21° C)
indoor temperature, 15 mph wind, and no solar load. The U-factor may be
expressed for the glass alone or the entire window, which includes the
effect of the frame and the spacer materials. The lower the U-factor,
the greater a window's resistance to heat flow and the better its
insulating value.

To convert the U-factor from US (imperial/IP) to European (metric/SI),
multiply the imperial number by 5.678. For example, If U=0.35 Btu/hr-sq
ft-°F in imperial units, then 0.35*5.678 = 1.9873. The U-factor in
metric units will be 1.9873 W/sq m-°K.

UBC. Uniform Building Code.

Ultraviolet light (UV). The invisible rays of the
spectrum that are outside of the visible spectrum at its
short-wavelength violet end. Ultraviolet rays are found
in everyday sunlight and can cause fading of paint
finishes, carpets, and fabrics.

V

Vapor retarder. A material that reduces the diffusion
of water vapor across a building assembly.

Vent. The movable framework or sash in a glazed
window that is hinged or pivoted to swing open.

Vinyl. Polyvinyl chloride material, which can be
both rigid or flexible, used for window frames.